1. Field of the invention
The present invention relates to a printing process unit for performing a printing operation by adhering toner to a printing medium by a photosensitive drum, and more particularly, to a printing process unit which is capable of collecting all toner remaining on the photosensitive drum after the printing operation to eliminate stains of the printing medium.
2. Description of the Related Art
A printing process unit of a printer such as a laser printer in which toner is adhered to a printing medium to perform printing operation, for example, is provided with a developing roller 92 and a cleaning roller 93 each at the front side and rear side of a photosensitive drum 91 for adhering toner to a printing medium S as shown in a sectional view of FIG. 6. The developing roller 92 serves to supply toner to the photosensitive drum 91 and the cleaning roller 93 serves to collect residual toner on the photosensitive drum 91. Provided above the photosensitive drum 91 is a transferring roller 94 which absorbs toner on the photosensitive drum 91 toward the printing medium S.
When the printing operation is performed in this printing process unit, the printing medium S passes through between the photosensitive drum 91 and the transferring roller 94, while a printing image is exposed by a laser beam at a point of P on the photosensitive drum 91. The exposed portion has a lower potential compared to the other portions, whereby an electrostatic latent image is formed. Further, the transferring roller 94 and the cleaning roller 93 are caused to keep a potential lower than that of the exposed image of the photosensitive drum 91. As a result, positive toner is supplied from the developing roller 92 at the position where the photosensitive drum 91 opposes to the developing roller 92, resulting in that the positive toner is adhered to the exposed image of the photosensitive drum 91 to form the electrostatic latent image.
When this portion reaches the position where the transferring roller 94 opposes to the printing medium S, toner on the photosensitive drum 91 is absorbed by the transferring roller 94 due to its low potential to adhere onto the printing medium S. Accordingly, the toner image is transferred to the printing medium S, and then, a fixing procedure is executed to the printing medium S to complete the printing operation. On the other hand, toner is somewhat remained on the photosensitive drum 91, however, this residual toner is collected by the cleaning roller 93 due to its low potential at the position where the photosensitive drum 91 opposes to the cleaning roller 93 for preventing the printing medium S from being stained when the medium S opposes again to the photosensitive drum 91.
When the printing operation is not performed, the potential of the cleaning roller 93 is kept higher than that of the photosensitive drum 91, and the potential of the transferring roller 94 is kept lower than that of the photosensitive drum 91. As a result, toner accumulated onto the cleaning roller 93 is removed to the photosensitive drum 91 to be collected by the transferring roller 94 due to its low potential at the position where the photosensitive drum 91 opposes to the transferring roller 94. This is because the toner accumulated onto the cleaning roller 93 is removed to be reused.
Considering the time when the printing process unit described above is performing the printing operation, the cleaning roller 93 has to collect all of the residual toner on the photosensitive drum 91. From this point of view, it is necessary that the whole width "b" of the cleaning roller 93 is the same as or more than the maximum width "a" which is set for supplying toner to the photosensitive drum 91 from the developing roller 92. When the width "b" is smaller than the width a, the residual toner may not be collected by the cleaning roller 93 to arrive again at the position where the photosensitive drum 91 opposes to the printing medium S, thereby being likely to stain the printing medium S. It is to be noted that the width "a" is smaller than the whole width of the developing roller 92 in FIG. 7, which is because toner is supplied to the developing roller 92 in the area of the width "a" from a toner supplying roller which is separately provided, and then the developing roller 92 supplies toner to the photosensitive drum 91.
On the other hand, considering the collecting of the residual toner on the cleaning roller 93 at the time when the printing process unit is not performing the printing operation, the developing roller 92 has to collect all toner removed to the photosensitive drum 91 from the cleaning roller 93. From this viewpoint, whole width "b" of the cleaning roller 93 has to be the same as or smaller than the developing width "a" of the developing roller 92 as shown in FIG. 8. When the width "b" is greater than the width "a", toner may be adhered to the outside of the width "a" of the developing roller 92, whereby it is likely to stain the portion other than the printing width of the printing medium S upon successive printing operation.
Both of the factors described above are contrary requirements. Further, it is impossible to strictly accord the width "a" with the width "b" in a real mass production process in which an allowance cannot be ignored, whereby it has been likely to occur that all toner are not collected due to this allowance to stain the printing medium S. There may be the case where one of the factors at the printing operation and non-printing operation is sacrificed to render the difference between the width "a" and the width "b" somewhat greater for assuredly preventing the stain caused by the other factor. However, the size (particularly width) of the printing process unit becomes unnecessarily large.